Hello,
I would like to propose adding literal syntax to allow creation of an empty set
without the need to call the type constructor. I believe the best choice for
such a literal, and one that has been proposed before, is `{,}`.
I know this idea has surfaced a few times before, but I cannot find any serious
consideration since the 3K ideas chain with the closest to a pronouncement here
https://mail.python.org/pipermail/python-3000/2006-May/001599.html. In that
thread the idea of a new syntax for the empty set did not seem to be rejected
outright, but more that the idea of set literals at all was still in question
and as empty sets were not implemented for 2.7 they were ultimately still left
out of 3k, intentionally or unintentionally.
Within cpython itself `set()` is used just over 200 times to assign a variable
the value of an empty set. As a comparison `dict()` is used only 15 times, and
all but one of those are within the test module, mostly for testing dict. On
the other hand, `{}` is used for assignment a little over 1300 times; I think
there is a clear preference for using literal over type construction when
possible.
I have a working implementation of this new syntax at
https://github.com/ucodery/cpython/tree/empty-set which includes a few changes.
Python from this branch allows `{,} == set()` but also `[,] == list()` and `(,)
== tuple`. Partly this was because the grammar changes were easier to do all
three but also because I personally liked keeping some symmetry between these
collection literals. The main point of starting this thread is to gather
opinions on adding `{,}` to the language, but I would like to discuss the other
two as well.
As a secondary point, I do think that this new tuple syntax would make teaching
python easier. Those new to python often believe that parentheses are what make
a tuple and only later, if ever, realize that it is only value(s) followed by a
comma that construct a tuple and that parentheses are places around it mainly
for readability. This misunderstanding often leads to a bug like `iter((4))`,
assuming the programmer meant `iter((4,))`. I believe this confusion about
whether it is the parentheses or the commas that construct a tuple is deepened
because two parentheses surrounding nothing _does_ specifically create the
empty tuple but `empty = ,` is a SyntaxError. With this literal tuple change,
those new to the language can be told that parentheses containing at least one
comma always creates a tuple (at least anywhere is is grammatically valid to
have a tuple).
Jeremy
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